I wrote recently with a question about mapping from a found token back
to that token's point in the input stream. I solved it on my own in
the meantime.

The solution was basically as follows:

- Maintain a pointer to the head of the string to be parsed (I am
using an API that inputs string data, not a file), "s_input"
- Maintain a running total of bytes, "n_offset", fed to the lexer as a
static variable updated within the input() function
- Use the variables in YY_CURRENT_BUFFER and do a little pointer
arithmetic to extract the desired information, as follows: